1. Field of the Invention
The present invention relates to an exhaust gas cleaning system for an internal combustion engine having an exhaust gas after-treatment device. Specifically, the present invention relates to the exhaust gas cleaning system for the internal combustion engine capable of estimating temperature of the exhaust gas after-treatment device and maintaining the exhaust gas after-treatment device in an optimum state, based on the estimated temperature.
2. Description of Related Art
Conventionally, as means for protecting the environment, various exhaust gas after-treatment devices for reducing air pollutants discharged from an internal combustion engine are proposed. For instance, catalyst systems such as an oxidation catalyst, a NOx removal catalyst and a three-way catalyst have been introduced progressively. Emission of particulate matters included in exhaust gas of a diesel engine is a problem to be solved. Therefore, introduction of a diesel particulate filter (DPF) or a DPF in which a catalyst is supported has been considered. The DPF collects the particulate matters and eliminates the collected particulate matters regularly by combustion.
Temperature control of the exhaust gas after-treatment device is an important factor for efficient operation of the device. Specifically, in the case where the catalyst is employed, the device should be used in a certain operating temperature range (for instance, 200–700° C.) in order to operate the device above a predetermined activation temperature, above which the catalyst is activated sufficiently, and meanwhile, to prevent damage to the catalyst caused by excessive temperature increase. For instance, the DPF is regenerated by heating the DPF above 600° C. with the use of unburned hydrocarbon, which is supplied by performing post-injection and the like. However, under some conditions, there is a possibility that the DPF may be heated excessively with combustion heat of the particulate matters. As a result, the catalyst may be degraded or a filter base material may be damaged.
A method for detecting a state of the catalyst by sensing the temperature of the after-treatment device is proposed, for instance, as disclosed in Japanese Patent Unexamined Publication No. H04-224221 (a first patent document) or Japanese Patent No. 2593506 (a second patent document). In the first patent document, temperature sensing means disposed in an exhaust gas cleaning system having a NOx reduction catalyst is disclosed. The exhaust gas cleaning system controls the temperature of the catalyst within a certain temperature range by changing an excess air ratio in accordance with the sensed temperature. In the second patent document, first and second temperature sensing means disposed upstream and downstream of the catalyst respectively are disclosed. Output signals from the first and second temperature sensing means are compared with each other in order to determine degradation of the catalyst.
The state of the exhaust gas after-treatment device such as the catalyst or the DPF is most suitably represented by temperature at a center of the device. However, as a matter of fact, it is difficult to directly sense the temperature at the center of the device. Therefore, in conventional devices, temperature of the exhaust gas upstream or downstream of the exhaust gas after-treatment device is regarded as the temperature of the device. For instance, in the first patent document, the temperature of the catalyst is controlled based on an assumption that the temperature of the exhaust gas flowing out of the catalyst coincides with the temperature of the catalyst. However, when a vehicle is accelerated or decelerated, the temperature of the exhaust gas flowing out of the catalyst does not coincide with the temperature of the catalyst necessarily. As a result, the temperature of the catalyst cannot be maintained in a proper range.
It is because a catalyst support or a filter base material is ordinarily formed of a ceramic honeycomb structural body, which has a large heat capacity. Since the catalyst support or the filter base material is formed of the ceramic honeycomb structural body, there is time lag since the temperature at the center of the device changes until the change in the temperature at the center of the device is reflected in the temperature of the exhaust gas downstream of the device. In addition, estimated temperature may deviate from actual temperature largely because of catalytic reaction or combustion of the particulate matters. In that case, there is a possibility that the catalyst may be degraded.
In the determination of the degradation of the catalyst disclosed in the second patent document, it is difficult to determine whether a difference between outputs of the two temperature sensing means is a temperature change caused by the catalytic reaction or a change in the temperature of the exhaust gas itself (a change in the temperature of the exhaust gas before entering the exhaust gas after-treatment device). It is because the temperature of the exhaust gas itself changes with the acceleration or the deceleration of the vehicle. Therefore, in that method, the degradation determination can be performed only while the vehicle is not traveling or while the vehicle is traveling at a constant speed. However, in the actual travel, the vehicle travels at a constant speed infrequently. Therefore, there is a practical problem in the above method.